The conventional method of locating a target which has been downed is by homing on a radio frequency beacon which emits a signal on any one of a number of frequencies reserved for that purpose. The radio frequency (RF) beacon normally obtains its power for operation from the target's own battery supply and radiates a signal from the target's existing telemetry antenna, which is switched to the locator beacon's circuitry upon application of the target's recovery system power.
A major disadvantage of the conventional system is that upon failure of the target's power supply, the RF recovery beacon will be without power to provide a homing signal. Since the RF beacon draws its battery current subsequent to the operation of the target's control functions, there is a high probability that the battery supply of the target will be unable to provide enough energy to properly operate the RF beacon. Furthermore, if the target should land upside down or if the target floats low in the water, the beacon antenna can be easily shorted such that it would not operate.
Underwater acoustic beacons have been manufactured which overcome many of the disadvantages and limitations of the RF beacons. However, acoustic beacons which are now commercially available are generally unsuitable as target recovery devices since they are normally not self-contained, are expensive to manufacture, in most cases only provide two frequencies of operation with only a single code or signature for identification purposes, require expensive crystal-controlled oscillators for stable frequency control, and are generally limited in range as well as requiring expensive battery power sources.